<!doctype html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta name="viewport"
        content="width=device-width, user-scalable=no, initial-scale=1.0, maximum-scale=1.0, minimum-scale=1.0">
  <meta http-equiv="X-UA-Compatible" content="ie=edge">
  <title>Document</title>
  <style>

    .my {
      font-size: 20px;
    }

    @media (max-width: 600px) {
      .my {
        font-size: 30px;
      }
    }
  </style>
</head>
<body>
<div class="my">My</div>
</body>
</html>